Respiratory nitrate reductase complexes will be studied in Staphylococcus aureus, a facultative fermenter, and in the denitrifier, Paracoccus (Micrococcus) denitrificans. The emphasis will be upon the role of membrane cytochromes in the formation, assembly and function of the complexes. The cytoplasmic and membrane forms of the nitrate reductase in S. aureus will be purified for comparison of subunit structure and enzymic properties. Membranes from heme-deficient mutants of S. aureus will be used for study of in vitro assembly of functional cytochrome and for investigation of the role of apocytochrome and heme prosthetic group in regulation of the nitrate reductase complex by oxygen repression and by nitrate induction. Nitrate reductase and associated cytochromes are to be purified from membranes of P. denitrificans for comparative studies with membrane components in mutant strains lacking reductase activity. Regulation of the nitrate reductase system in the denitrifier by oxygen repression and by nitrate and nitrite will be studied in wild types and mutant strains.